A&A 574, A123 (2015) Astronomy DOI: 10.1051/0004-6361/201423830 & c ESO 2015 Astrophysics Pre-hibernation performances of the OSIRIS cameras onboard the Rosetta spacecraft S. Magrin1;2, F. La Forgia2, V. Da Deppo3, M. Lazzarin2, I. Bertini1, F. Ferri1, M. Pajola1, M. Barbieri1, G. Naletto1;4, C. Barbieri1;2, C. Tubiana5, M. Küppers6, S. Fornasier7;8, L. Jorda9, and H. Sierks5 1 Centro di Ateneo di Studi e Attivitá Spaziali “Giuseppe Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy e-mail: [email protected] 2 Department of Physics and Astronomy, University of Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy 3 CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy 4 Department of Information Engineering, University of Padova, via Gradenigo 6, 35131 Padova, Italy 5 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany 6 European Space Astronomy Centre, ESA, Villanueva de la Cañada, 28691 Madrid, Spain 7 LESIA, Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris Diderot, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France 8 Université Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris, France 9 Laboratoire d’Astrophysique de Marseille, CNRS and Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France Received 18 March 2014 / Accepted 6 December 2014 ABSTRACT Context. The ESA cometary mission Rosetta was launched in 2004. In the past years and until the spacecraft hibernation in June 2011, the two cameras of the OSIRIS imaging system (Narrow Angle and Wide Angle Camera, NAC and WAC) observed many different sources. On 20 January 2014 the spacecraft successfully exited hibernation to start observing the primary scientific target of the mission, comet 67P/Churyumov-Gerasimenko. Aims. A study of the past performances of the cameras is now mandatory to be able to determine whether the system has been stable through the time and to derive, if necessary, additional analysis methods for the future precise calibration of the cometary data. Methods. The instrumental responses and filter passbands were used to estimate the efficiency of the system. A comparison with acquired images of specific calibration stars was made, and a refined photometric calibration was computed, both for the absolute flux and for the reflectivity of small bodies of the solar system. Results. We found a stability of the instrumental performances within ±1.5% from 2007 to 2010, with no evidence of an aging effect on the optics or detectors. The efficiency of the instrumentation is found to be as expected in the visible range, but lower than expected in the UV and IR range. A photometric calibration implementation was discussed for the two cameras. Conclusions. The calibration derived from pre-hibernation phases of the mission will be checked as soon as possible after the awak- ening of OSIRIS and will be continuously monitored until the end of the mission in December 2015. A list of additional calibration sources has been determined that are to be observed during the forthcoming phases of the mission to ensure a better coverage across the wavelength range of the cameras and to study the possible dust contamination of the optics. Key words. instrumentation: detectors – space vehicles: instruments – techniques: photometric 1. Introduction In Sect.2 we therefore study the past performances of the two cameras, the Narrow Angle and Wide Angle Camera (NAC The ESA Rosetta mission was launched on 2 March 2004, and and WAC), through the observations of calibration stars. In par- since then, the Optical, Spectroscopic, and Infrared Remote ticular, we investigate the past responses of calibrator signal Imaging System (OSIRIS, Keller et al. 2007), which is a two- through time with different filters to be able to evaluate possible camera system, performed many different scientific observa- aging effects on the optics and to determine whether the pho- tions. It acquired data during a swing-by with Mars and Phobos tometric requirements set by the original mission science goals in 2007, a fly-by with asteroid (2867) Steins in 2008, two swing- have remained stable. We also inspect the dynamical range of the bys with Earth in 2007 and 2009, and a fly-by with asteroid two CCDs by checking the linear dependence between the stellar (21) Lutetia in 2010, to mention only the main activities. On signal and the exposure time of observations. The transmission 8 June 2011, the spacecraft entered into deep-space hiberna- and reflectivity curves of each component of the two cameras, tion mode. It successfully exited hibernation on 20 January 2014 measured during on-ground calibration tests, is used to estimate for the cruise to the rendez-vous with the primary scientific tar- the efficiency of the instrumentation. We thus check and com- get of the mission: the Jupiter-family comet 67P/Churyumov- pare the expected calibrator signals that are deduced from these Gerasimenko. instrumental efficiency curves and from the observed one, which Establishing the stability of the instrument through time is is computed from images acquired in flight. vital to performing a correct absolute flux calibration of the Following this investigation, we present in Sect. 3 the robust cometary data. formulation for the absolute flux calibration of OSIRIS images. Article published by EDP Sciences A123, page 1 of 13 A&A 574, A123 (2015) Table 1. Basic parameters of the NAC and WAC system and CCD specification (Keller et al. 2007). NAC WAC Optical design 3-mirror off-axis 2-mirror off-axis Detector type 2k × 2k CCD 2k × 2k CCD Angular resolution (µrad px−1) 18.6 101 Focal length (mm) 717.4 140 (sag)/131 (tan) Field of view (◦) 2:20 × 2:22 11:35 × 12:11 Telescope aperture 6:49 × 10−3 m2 4:91 × 10−4 m2 F-number 8 5.6 Spatial scale from 1km (cm px−1) 1.86 10.1 NAC and WAC CCD Pixel size 13:5 × 13:5 µm2 Full well >120 000 e− px−1 System gain ≈3.1 e−/DU Readout noise (CCD) ≈15 e− rms Dark charge generation <0.1 e− s−1 px−1 @180 K ≈400 e− s−1 px−1 @293 K – (with dithering) Readout rate 1.3 Mpx s−1; 650 kpx s−1 per channel Readout time (full frame) 3.4 s (2 channels) Table 2. NAC and WAC filter names and wheel positions. plate – visible (NFP-Vis) with respect to far focus plate – visible (FFP-Vis) is also planned, which will allow optimizing the res- NAC WAC olution of the NAC for observations of the comet surface at a Position Wheel 1 Wheel 2 Wheel 1 Wheel 2 distance shorter than 4 km. 1 FFP_UV FFP_IR empty empty We can therefore define the filters as twins, which can be ob- 2 FFP_Vis Orange Green Red tained by combining different clear filters in addition to the same 3 NFP_Vis Green UV 245 UV 375 band-pass filter. These combinations share the same passband, 4 Near-IR Blue CS CN but have different efficiencies in general because clear filters 5 Ortho Far-UV UV 295 NH2 have different efficiencies. Together with defining twin combina- 6 Fe2O3 Near-UV OH Na tions, it is also possible to define a baseline filter combination by 7 IR Hydra UV 325 OI selecting a representative of the group and deduce the flux cali- 8 Neutral Red NH Vis 610 bration of the whole group according to the baseline calibration. Notes. Position of the two filter wheels of all filters for NAC and WAC. In AppendixA we list and define the physical parameters In principle, every combination is possible, but only some of them are and units we used. useful for scientific purposes. 2. Performance of OSIRIS cameras Section4 is dedicated to the description of the conversion be- 2.1. Expected count rate tween the signal observed on the surface of solar system bodies −1 The expected total count rate, Kx;y in DN s , of a given source and of their correct geometric reflectance. on an image acquired through the (x; y) filter combination is The NAC is a three-mirror anastigmat system, while the expressed by WAC has an off-axis optical configuration obtained with two as- Z 1 n pherical mirrors (Keller et al. 2007). In Table1 we report the Kx;y = '(λ) · M (λ) · Fx;y(λ) · TARP(λ) main characteristics of the NAC and WAC system, together with 0 the specifications of the CCDs. λ 1 × Q(λ) · · · AP dλ, (1) A plane-parallel antireflection coated plate, referred to as hc IG antiradiation plate (ARP), was added in front of the CCD for where radiation shielding. Both cameras are equipped with two filter wheels placed in – '(λ) is the spectral irradiance of the source at the camera entrance aperture, expressed in W m−2 nm−1; front of the CCD. A passband is defined by a combination of two − filters, hereafter (x; y), where x is the filter position on the first – Q(λ) is the CCD quantum efficiency expressed in e =ph; wheel and y is the position on the second wheel. In Table2 we – IG is the inverse gain, which hereafter is taken to be 3:1 e−=DN; list all filters available for the two cameras by specifying their n position on the wheels. – M (λ); Fx;y(λ); TARP(λ) are the mirror spectral reflectance The WAC filters are located in seven of the eight positions (through n mirrors: n = 2 for WAC or n = 3 for NAC), filters, of the two filter wheels, from 2 to 8, leaving the first position in and antiradiation plate (ARP) transmissions as a function of the wavelength; both wheels empty.